So the latest MacOS Update broke the gpg-mail Plugin again. But it seems that the old UUID Trick saves the show.
First close Mail.app.
Then you need to find out the new PluginCompatibilityUUIDs from Mail.app and the Message Framework.
cat /System/Library/Frameworks/Message.framework/Resources/Info.plist | grep UUID -A 1 cat /Applications/Mail.app/Contents/Info.plist |grep UUID -A 1 | grep UUID -A 1
Then you need to open the gpg-mail Bundle.
Look into /Users/[username]/Library/Mail/Bundles
You can open GPGMail.mailbundle via the Context Menu. You need to add the two new UUIDs to the Node SupportedPluginCompatibilityUUIDs in the Info.plist file.
Save and then restart Mail.app.